Substantial evidence exists suggesting that calmodulin (CaM) and calcium regulation of essential processes in normal cells may be perturbed upon cellular transformation. Our research has clearly demonstrated that normal and transformed cells differ in their abilities to regulate CaM levels. Normal cells both up and down regulate calmodulin in response to culture density--presumably reflecting cell surface contact-mediated regulatory processes. Transformed chick embryo fibroblasts, on the other hand, appear to be altered in this regulation. It is clear that calmodulin controls numerous cell-cell and cell-substratum interactions, particularly those involved in filament-based motility and cytoskeletal systems. The additional fact that these systems also may have essential functions in the cell growth process and appear to be altered on transformation provides a clear rationale for future work to investigate alteration of CaM and its target proteins. Such studies are made possible by the development in this laboratory of specific, modified calmodulin probes to be used for both in vitro and in vivo photochemical cross-linking for identification and isolation of specific CaM-binding proteins. A series of studies is outlined to complete our investigations of the regulation of calmodulin levels in normal cells. Further studies will examine CaM binding to specific proteins as a function of cell cycle and perturbation in the expression of these target proteins, as a consequence of cellular transformation. These studies will include careful examination of enzymes such as myosin light chain kinase, the calcium-dependent phosphatase calcineurin, caldesmon (which has been implicated in the regulation of actin-filament formation) and various metabolic enzymes which calmodulin is known to regulate. These studies will be aimed toward definition of transformation-dependent alterations in specific enzyme systems that may account for the changes in cellular morphology and metabolic state characteristic of the transformed phenotype.